The single ones easy, but the L shaped one a little more tricky. You need some 25mm cellotex, and you'd put a 2x1 batten at the top and bottom of the units, and then put the cellotex in between the battens. For neatness, you could also put some battens at the side of the units as well. The battens will support the wall units, so must be fixed securely to the walls, and they can be set so that they don't protrude around the units. For the L shaped units, you need to batten out both walls, and that will shift both sides along an inch, which is fine as long as nothings in the way.